This widget is used to configure the natural light on KA1/H2O2. It
allows the user to set the gain/offset values for white/red/green, as
well as the exponent used in the formula.
Numbers can be changed via screen keyboard or through the plus/minus
buttons next to the numbers (holding the buttons will in/decrase with
smaller steps). Changes are applied immediately, but will only be set
permanently if 'OK' is pressed. If 'Cancel' is pressed, the new values
are discarded and the old ones restored. Also, pressing 'OK' will save
these values in the koreader settings file (there's no support for
changing these values in Nickel, so there's nothing to sync to its
config file). The 'Restore Default' button will install the original
defaults that were derived from Nickel's behavior.
This widget is meant to be called from the frontlight widget and is
placed at the top, so that the frontlight widget can still be used to
test the current values (see next commit).
